U.S. patent number 6,035,933 [Application Number 08/995,141] was granted by the patent office on 2000-03-14 for process for the thermo-hydraulic control of gas hydrates.
This patent grant is currently assigned to Petroleo Brasileiro S.A.-Petrobras. Invention is credited to Carlos Nagib Khalil, Lucia Cristina Ferreira Leite, Nelson De Oliveira Rocha.
United States Patent |
6,035,933 |
Khalil , et al. |
March 14, 2000 |
Process for the thermo-hydraulic control of gas hydrates
Abstract
A process for the thermo-hydraulic control of gas hydrates in
subsea production and injection wells as well as pipelines which
transport liquid or gaseous hydrocarbons is described, the process
making use of a Nitrogen Generating System foamed or in solution.
The control may signify the prevention of the formation of the gas
hydrates or the dissolution of the gas hydrate plug already formed.
Under conditions of use designed for the prevention of the
formation of gas hydrates the SGN fluid prevents the thermal
conditions leading to the gas hydrate plugs. Under conditions of
use designed to dissolve the already formed gas hydrate plugs, the
SGN fluid alters the thermo-hydraulic conditions which favor the
gas hydrate plugs, so that they are dissolved and return to the
water+gas phase.
Inventors: |
Khalil; Carlos Nagib (Rio de
Janeiro, BR), Rocha; Nelson De Oliveira (Rio de
Janeiro, BR), Leite; Lucia Cristina Ferreira (Rio de
Janeiro, BR) |
Assignee: |
Petroleo Brasileiro
S.A.-Petrobras (Rio de Janiero, BR)
|
Family
ID: |
4068116 |
Appl.
No.: |
08/995,141 |
Filed: |
December 19, 1997 |
Foreign Application Priority Data
|
|
|
|
|
Oct 17, 1997 [BR] |
|
|
9705076 |
|
Current U.S.
Class: |
166/263; 137/13;
166/300; 166/310; 166/401; 210/696; 166/309; 166/312; 166/371;
507/277; 507/90; 507/927; 585/950; 507/216; 166/357; 166/302 |
Current CPC
Class: |
C09K
8/52 (20130101); Y10T 137/0391 (20150401); Y10S
585/95 (20130101); Y10S 507/927 (20130101); C09K
2208/22 (20130101) |
Current International
Class: |
C09K
8/52 (20060101); E21B 043/22 (); E21B 043/24 ();
E21B 043/34 () |
Field of
Search: |
;137/13
;166/263,267,272.6,275,279,300,302,305.1,309,310,312,357,371,401,402
;210/696 ;507/90,216,277,927 ;585/15,899,950 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"SGN Technology, The Environmentally Sound Solution for Organic
Buildups", Petrobras/Maritima (undated)..
|
Primary Examiner: Suchfield; George
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas, PLLC
Claims
We claim:
1. A process for the thermo-hydraulic control of gas hydrates under
conditions of gas hydrate formation within a petroleum well, which
comprises the following steps:
a) preparing in one single mixing vessel an aqueous, equimolar
solution of ammonium chloride and sodium nitrite of concentration
between 2.0 and 4.5 molar;
b) viscosifying the aqueous solution of step a) with hydroxyethyl
cellulose of a concentration between 0.3 to 1.2 weight/volume
keeping pH at values between 8.0 and 8.3;
c) in a separate vessel, preparing a solution of acetic acid
activator of 40% volume;
d) adding between 1 and 2% volume of activator taken from said
separate vessel to aqueous, viscosified solution taken from said
one single mixing vessel so as to obtain a foamed and viscosified
nitrogen gas fluid;
e) pumping the nitrogen gas- and heat-generating foamed and
viscosified fluid of step d) through a flexitube positioned in the
interior of the production string so as to contact said fluid with
the gas hydrate and thus cause the gas hydrate to dissolve as a
consequence of the increase in temperature and reduction of
pressure;
f) recovering the spent fluids and separating the aqueous phase in
an adequate separator.
2. A process according to claim 1, wherein the petroleum well is a
production well.
3. A process according to claim 1, wherein the gas hydrate is
formed above a Wet Christmas Tree of a petroleum production
well.
4. A process according to claim 1, wherein the gas hydrate is
formed below a Wet Christmas Tree of a petroleum production
well.
5. A process according to claim 1, wherein the foamed and
viscosified nitrogen gas fluid of step d) is formed in a pipeline
attached to a petroleum explotation system.
6. A process according to claim 1, wherein the foamed and
viscosified nitrogen gas fluid of step d) is formed in a pipeline
for the transport of liquid fluids and gaseous fluids.
7. A process for the thermo-hydraulic control of gas hydrates under
conditions of gas hydrate formation within a petroleum well, which
comprises the following steps:
a) preparing in one single mixing vessel an equimolar, aqueous
solution of ammonium chloride and sodium nitrite of concentration
between 0.5 and 4.5 molar;
b) separately preparing a solution of acetic acid activator at 40%
volume;
c) adding between 1 and 2% volume of the activator of step c) to
the aqueous solution of step a) so as to obtain a nitrogen- and
heat-generating fluid in solution;
d) through reservoir perforations, pumping the nitrogen- and
heat-generating fluid of step c) to the interior of a reservoir so
as to cause the heating of the reservoir;
e) allow the nitrogen- and heat-generating fluid a sufficient soak
time so as to prevent the formation of gas hydrate plugs as a
consequence of the increase in temperature at the same time as
natural gas is injected;
f) withdraw the spent fluids.
8. A process according to claim 7, wherein in step e), the heating
with the nitrogen- and heat generating fluid is also designed to
dissolve an already formed gas hydrate.
9. A process according to claims 7 or 8, wherein the heating with
the nitrogen- and heat-generating fluid is designed to control the
gas hydrates in an injection petroleum well.
10. A process according to claims 7 or 8, wherein the heating with
the nitrogen- and heat-generating fluid is effected in a petroleum
reservoir.
11. A process according to claim 6, where said pipeline is a
pipeline for the transport of fluids comprising petroleum oil.
12. A process according to claim 6, where said pipeline is a
pipeline for the transport of fluids comprising light hydrocarbon
gas.
Description
FIELD OF THE INVENTION
The present invention relates to a process for the thermo-hydraulic
control of gas hydrates with the aid of a Nitrogen Generating
System (also known as "SGN", based on the Portugese "Systema
Gerador de Nitrogenio"). More specifically, the present invention
relates to a process for the thermo-hydraulic control of gas
hydrates of any low boiling hydrocarbon stream in the presence of
humidity so that the thermo-hydraulic conditions lead to the
formation of such gas hydrates, the process being carried out with
the aid of Nitrogen Generating System (SGN).
BACKGROUND INFORMATION
Gas hydrates are formed whenever water and hydrocarbon gases are
combined under high pressure and low temperature.
Gas hydrates are crystal lattices made up of two or more
constituents. The molecules of one component (always water) form a
structure having relatively large cavities, which are occupied by
the molecules of other constituents, these being separate gases or
gaseous mixtures.
Gases which are important from the industrial as well the
laboratory aspect show structures defined by the formula X.nH.sub.2
O where X is the hydrate-forming molecule while the number of water
molecules in the compound is n>5.67. Generally, hydrates are
formed only in the presence of condensed water, that is, liquid
water or ice. The water molecules linked by hydrogen bridges form a
host (receiving) network around one or more species of the guest
molecules. A physical encapsulation process occurs which is
accompanied by weak interactions between the host-guest
constituents when the guests enter the cavities of the host
structure and are released therefrom under appropriate
circumstances, by the collapse of the host structure.
Thus, the gaseous components within the cavities are not directly
linked to the water molecules of the network. Due to geometrical
reasons, such components cannot abandon the network of water
molecules linked by hydrogen bridges until such network
collapses.
Therefore, in the stable state, gas hydrates are always clathrate
compounds of two or more components, since the components are
mutually inserted via a complex mechanism. However the cohesion
forces between the host and guest molecules do not suffice for
forming a clathrate. Besides the cohesive forces, two basic
criteria must be met in order to form a clathrate: the trend of
water molecules to form a network must be satisfied, while the
guest molecules must show suitable size and shape to enter the
cavities of the hydrogen-bridged water network. A further
requirement for forming the structure is that there should not be
any chemical reaction between the guest molecules and the water
molecules, that is, during crystallization, hydrolysis as well as
hydration should be avoided in order to prevent a structure whose
total energy would be lower than that of the clathrate.
Generally, it is considered that for the gas hydrates to occur the
components or constituents should meet the following requirements:
low solubility in water, sufficient volatility, homopolar
character, not too large van der Waals forces, evaporation heat
lower than 31,400 J mol.sup.-1 as well as boiling point lower than
60.degree. C., the hydrate-forming component being devoid of
hydrogen atoms able to yield additional hydrogen bridges. Finally,
the hydrate-forming gas should not be fairly soluble in either
water, as are for example NH.sub.3 or HCl, or a water miscible
liquid, for example CH.sub.3 OH.
Studies carried out in the field of gas hydrates indicate that the
initial conditions for forming gas hydates are determined by the
nature of the gas, the water state, the pressure and the
temperature. The formation conditions are set in heterogeneous
phase diagrams plotted as pressure vs. temperature.
The probability that a gas hydrate will be formed is as high as its
stability. The stability degree of a gas hydrate and consequently
its dissociation temperature are influenced by the molecular size
and the geometric shape of the hydrate-forming components. Among
the hydrocarbon hydrates, the more stable are those of propane and
isobutane. The conditions for hydrate formation for a single- or
multicomponent gaseous system are thus more or less altered by the
presence of a third component. Generally it can be said that this
effect depends on the gas composition, the density of the
corresponding gas, the nature and amount of the substance which is
altering the structural conditions in water, and on the pressure
existing in the system. In the presence of electrolytes or polar
solutes, the primary factors which act to alter the conditions of
hydrate formation and dissociation are the structural variations
which depend mainly on the solute pressure, temperature and
composition and also on the energy variations of the interactions
among molecules.
Researches has shown that any amount of electrolytes dissolved in
water will lower the temperature of hydrate formation at a given
pressure. In low amounts, alcohols increase the temperature of
hydrate formation; however, for increasing amounts, such
temperature is lowered. In this latter case it is hypothesized that
structural cavities in water are partially occupied (for example,
by methyl groups in the case of methanol) and thus an ordering of
the hydrocarbon chains similar to that of ice is enhanced in the
vicinity of the organic molecules. For higher amounts of alcohol,
the clathrate-forming aggregates are broken, whereby the
possibility of hydrate formation is decreased in the same way as in
the case where the water structure is adversely affected by the
presence of electrolytes. The inhibiting effect of electrolytes and
alcohols is very important in the processes of production and
transportation of natural gas, and may be extended to other
processes as well.
Gas hydrates frequently occur during working out of subsea wells,
mainly in deep-water wells. The gas hydrate deposits are mainly
made up of petroleum gas and formation water or aqueous fluids
generated by combined effects of turbulence, pressurization and
cooling.
When the gas hydrate deposits are found in the production string or
even in the surgency line, such deposits invariably cause the
complete plugging of the production flow.
Under conditions of secondary recovery such as the method known as
Water Alternating Gas (WAG) where water and gas are alternatively
pumped into a reservoir through an injection well under conditions
of low temperature and high pressure, the water-as mixture may form
hydrates which can plug the injection well, bringing huge drawbacks
to the well infectivity. It is then interesting to prevent the
formation of these hydrates by heating the reservoir with the aid
of the SGN of the present invention.
Also, under conditions of petroleum oil production, there are
situations where the gas produced in the presence of cold water
creates conditions of gas hydrate formation, which may plug the wet
gas streamflow.
Still, the transportation of petroleum fluids along pipelines or
lines from offshore equipments to shore facilities may generate
conditions for the formation of gas hydrates, the flow of fluid
throughout the pipeline or line being thus impaired.
In the natural gas industry the occurrence of gas hydrates is met
on a day-by-day basis, since the thermo-hydraulic conditions for
such are highly favored.
Therefore, various thermodynamic conditions are found which favor
the occurrence of gas hydrates, in production as well as in the
secondary recovery of oil as well as in the transportation of
petroleum fluids, besides situations which can be found in the
production of natural gas from petroleum reservoirs.
The usual practice to prevent gas hydrate formation is the addition
to the aqueous fluid of an anti-freezing agent in amounts of 10 to
40% vol. Normally such agents are hydroxylated compounds such as
primary alcohols in C.sub.1 -C.sub.4, besides glycols. In Brazil
ethyl alcohol is usually employed, with good results and relatively
low cost.
U.S. Pat. No. 5,460,728 teaches a process for the inhibition of the
formation of gas hydrate in streams which contain low boiling
hydrocarbons and water, these streams being displaced throughout a
conduit or pipeline. The process comprises adding to the stream a
nitrogen component in a sufficient amount to inhibit the formation
of gas hydrates in the mixture at the temperature and pressure
found in the conduit.
U.S. Pat. No. 5,232,292 teaches a process for the control of
clathrate hydrates in fluid systems, the hydrates hindering the
flow of fluid in a fluid system. The process comprises the contact
of an additive with the clathrate mass. Preferably, the additive
contains a cyclic chemical group having five, six and/or seven
members. The additives include a poly(N-vinyl lactam) having
molecular weight higher than 40,000, the polymer comprising a
backbone, a first cyclic chemical grouping which extends from the
backbone, and a second cyclic grouping extending from the backbone,
the first cyclic grouping comprising a nonaromatic five-member
organic heterocyclic ring having an internal amide, the second
cyclic chemical grouping comprising a nonaromatic seven member
organic heterocyclic ring having an internal amide, the polymer
comprising a non-cyclic chemical group extending from the backbone.
Representative polymers are N-vinyl pyrrolidone and hydroxyethyl
cellulose, used alone or in combination.
U.S. Pat. No. 5,244,878 teaches a process for delaying and/or
reducing the agglomeration tendency of hydrates in conditions under
which a hydrate may be formed, which comprises adding to the
hydrate-forming stream of gas and water an amphiphilic non-ionic
compound chosen among the group of polyol esters and substituted or
non-substituted carboxylic acids. The amphiphilic compound may be
also an anionic amphiphilic compound.
U.S. Pat. No. 5,076,364 teaches a process for preventing gas
hydrate formation in a gas well by injecting a carrier and an
alcohol such as glycerol or a glycerol derivative into the well and
connected facilities/pipelines.
U.S. Pat. No. 4,856,593 teaches, in a process for flowing through a
pipeline a wet gas stream from an offshore producing well to shore
under conditions of temperature and pressure conducive to the
formation of gas hydrates, an improvement which comprises
introducing in the wet gas stream a surface active agent of the
group of organic phosphonates, phosphate esters, phosphonic acids,
salts and esters of phosphonic acid, inorganic polyphosphates,
esters of inorganic polyphosphates, polyacrylamides and
polyacrylates in a sufficient amount to prevent stoppage of the
flowing stream.
However, the control of gas hydrate formation by means of additives
may be costly and of reduced efficacy.
On the other hand, the use of nitrogen gas and heat for various
applications is well-known.
U.S. Pat. No. 4,846,277, of the Applicant and hereby fully
incorporated as reference, teaches a continuous process for the
hydraulic fracturing of a well with in situ nitrogen foam
generation from the exothermic reaction between nitrogen inorganic
salts, chiefly ammonium chloride and sodium nitrite, in the
presence of a buffer which is able to keep the pH solution at 5.0
or less, and a viscosifying compound which may be any hydrosoluble
polymer or gel which is able to increase the effective viscosity of
the generated foam. The buffer system may be acetic acid at
concentrations of from 0.5 volume % and the viscosifying compound
is preferably hydroxyethyl cellulose (HEC). The polymeric
viscosifier shows various advantages relative to the usual surface
agents, since those may alter the rock wettability, emulsify when
contacted with oil or precipitate if incompatible with the
formation water. Further, the amount of polymeric viscosifier is
less than that of surface agent for the same viscosifying
degree.
U.S. Pat. No. 5,183,581 of the Applicant and hereby fully
incorporated as reference, teaches a process for the dewaxing of
producing formations based on a Nitrogen Generating System/Emulsion
designed for the dewaxing of producing formations with the aid of
nitrogen gas and heat generated by the reaction between aqueous
solutions of nitrogen inorganic salts in the presence of an
emulsified organic solvent. Paraffin deposits are typically made up
of preferably linear, saturated hydrocarbon chains in C.sub.16 to
C.sub.80 in admixture with branched hydrocarbons, asphaltenes,
water and various mineral substances. The deposition phenomenon or
precipitation of solid wax is an example of fluid/solid phase
equilibrim, which can be explained in the light of principles of
solution thermodynamics, that is, the solution of a hydrocarbon of
higher molecular weight in hydrocarbons of lower molecular weight
which function as solvents. That is, high molecular weight solids
precipitate whenever the transport ability of the compound which
works as solvent for the fluid is reduced.
U.S. Pat. No. 5,580,391 of the Applicant and hereby fully
incorporated as reference teaches a process for the thermo-chemical
cleaning of storage tanks which contain sludges from petroleum oil
or related products. The process is carried out by the combined
action of an organic solvent and the generation of nitrogen gas and
heat, whereby is produced heating in situ, agitation by turbulence
and flotation of the fluidized sludge, which after being collected
and transferred to tanks or desalting units can be reintroduced in
the usual refining flow.
U.S. Pat. No. 5,539,313 of the Applicant and hereby fully
incorporated as reference teaches a process for the thermo-chemical
dewaxing of hydrocarbon transmission conduits, which comprises
treating the wax-containing conduit with a water-in-oil emulsion,
co-currently to the production flow. The emulsion contains
inorganic reactants which generate nitrogen and heat, which
fluidize the paraffin deposit which is later driven off by cleaning
beds.
The literature thus indicates on the one hand efforts for fluidize
the gas hydrates by incorporating an additive to the oil or gas
stream so as to alter the thermo-dynamic conditions of hydrate
formation. On the other hand, the Applicant has developed a
nitrogen and heat-generating treating fluid--the SGN fluid--which,
by generating nitrogen and heat can possibly alter the
thermo-hydraulic hydrate-forming conditions so as to prevent the
formation or dissolve the hydrates which may form in producing
wells, injection wells or reservoirs, as well as those formed in
gas conduits submitted to conditions of hydrate formation.
SUMMARY OF THE INVENTION
The present invention relates to a process for the thermo-hydraulic
control of gas hydrates which may form from hydrocarbon gases of
low boiling point, for example hydrocarbons in C.sub.1 -C.sub.7
brought into contact with water, under thermo-hydraulic conditions
conducive to hydrate formation, wherein a foamed fluid based on an
aqueous solution of nitrogen salts designed to generate nitrogen
and heat, the aqueous solution being viscosified with the aid of
high-molecular weight, non-ionic cellulose polymer, is made to
contact the hydrate in order to dissolve it. In situ foam
generation with simultaneous release of heat and lowering of
hydrostatic pressure of the system alters the thermo-hydraulic
conditions to which the hydrate is submitted, providing for the
dissolution or dissociation of the hydrate into water and gas.
Under somehow different conditions for the control of gas hydrates
there is no need to viscosify the SGN system, it being then applied
as a nitrogen- and heat-generating solution.
The basic concept of the present invention comprises the in situ
generation of foam with simultaneous release of heat and/or
hydrostatic pressure reduction of fluid-containing pipes whereby
the thermo-hydraulic conditions to which the hydrate is submitted
are altered, so as to prevent the formation of, or dissolve the
hydrate in its constituents, that is, water and gas.
The process of the present invention for the thermo-hydraulic
control of gas hydrate formation in a producing well comprises, for
the SGN/Foam mode, the following steps:
a) based on kinetic reaction studies, determining the concentration
of nitrogen salts necessary to prepare the nitrogen- and
heat-generating solution to be contacted with the gas hydrate:
b) preparing in one single vessel an equimolar aqueous solution of
ammonium chloride and sodium nitrite of concentration between 2.0
and 4.5 molar determined according to step a);
c) viscosifying the salt solution of b) with hydroxyethyl cellulose
at a concentration between 0.3 to 1.2% weight/volume keeping the pH
between 8.0 and 8.3;
d) separately preparing a solution of acetic acid activator at 40%
volume;
e) adding between 1 and 2 volume % of activator of step d) to the
viscosified aqueous solution so as to generate nitrogen gas fluid
foamed and viscosified;
f) pumping the foamed and viscosified nitrogen- and heat-generating
fluid of step e) through a flexitube to as to contact the foamed
and viscosified fluid with the gas hydrate and dissolve the gas
hydrate by the increase in temperature and pressure reduction;
g) recovering the spent fluids and separating the aqueous phase in
a separator.
In the SGN/Solution mode, the treating fluid comprises a solution
of the nitrogen- and heat-generating salts added only of the
necessary amount of acetic acid and injected into a well submitted
to a Water Alternating Gas working.
In pipelines which transport light hydrocarbons where a wet stream
is under hydrate-forming conditions the SGN/Solution treating fluid
is injected through the pipeline or conduit, the contact of the
treating fluid and the flowing fluid altering the hydrate-forming
thermo-hydraulic conditions so as to dissolve the hydrates and
restore flow within the pipeline.
Therefore, the present invention provides for a Nitrogen Generating
System for the thermo-hydraulic control of gas hydrates formed
during petroleum oil production from subsea wells, mainly
deep-water wells.
The present invention provides further for a Nitrogen Generating
System for preventing the formation of gas hydrates during water
injection in subsea wells.
Also, the present invention provides for a Nitrogen Generating
System for the thermo-hydraulic control of gas hydrates formed in
conduits or pipelines which transport petroleum fluids from
offshore to shore installations.
The present invention contemplates further the control of gas
hydrates formed from light hydrocarbon gases such as natural gas
and water, under conditions which favor the formation of hydrates,
the hydrocarbon gases being transported in conduits or
pipelines.
Therefore the present invention provides for a process based on
Nitrogen Generating System for the thermo-hydraulic control of gas
hydrates formed under thermo-hydraulic conditions which favor the
formation of gas hydrates, these being formed from C.sub.1 -C.sub.7
hydrocarbon gases and water, these hydrates occurring during
production, injection or transport operations of petroleum fluids
as well as in operations related to natural gas.
The present invention provides for a process for the control of gas
hydrates already formed or whose potential formation is indicated
in view of the combination of thermo-hydraulic conditions which
favor the building up of these hydrates.
The present process, such as described and claimed in the present
application, is not described nor suggested in the literature.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a phase diagram of the gas hydrate submitted to
SGN/Foam.
FIG. 2 is a simplified flowsheet of the mode of the present
invention designed for the thermo-hydraulic control of gas hydrates
by means of SGN/Foam.
FIG. 3 is a schematic illustration of the labscale physical
simulator used for simulating the in situ foam generation according
to the present invention.
FIG. 4 is a simplified flowsheet of the mode of the present
invention designed for the thermo-hydraulic control of gas hydrates
by means of SGN/Solution.
FIG. 5 is a phase diagram of a gas hydrate submitted to the
SGN/Solution mode of the invention.
DETAILED DESCRIPTION--PREFERRED MODES
In the present specification, the expression "gas hydrate control"
means either the dissolution of the gas hydrate plugs already
formed or the prevention of their formation.
According to the SGN method, the nitrogen and heat generation is
effected by the reaction of nitrite and ammonium ions present in an
aqueous solution of these salts, from which are obtained nitrogen
gas and heat. The nitrogen- and heat-generating aqueous solution
contains: a) a compound which contains at least one atom of
nitrogen to which is linked at least one hydrogen atom, such
compound being able of being quick and exothermically oxidized, in
an acid aqueous solution, so as to yield heat, nitrogen gas and
by-products which are liquid or dissolved, while substantially
inert to the well or to any equipment which these products may be
contacted; b) at least one oxidizing agent able to oxidize the
nitrogen compound of a); c) a buffer system able to keep the
solution pH at a level around 5.0 or less. Such as applied in the
present invention, the reaction system may further comprise a
viscosifying agent which may be any hydrosoluble polymer or gel
which is able to increase the effective viscosity of the generated
foam.
In other applications, SGN may be applied solely as an aqueous
solution, without the addition of any viscosifying nor surface
active agent.
In spite of the fact that various oxidation-reduction couples may
be used for preparing the nitrogen- and heat-generating aqueous
solution of the present invention, such as urea-sodium
hypochlorite, urea-sodium nitrite, ammonium chloride and sodium
hypochlorite or ammonium chloride-sodium nitrite, this last one is
the preferred couple to be used in the present process for the
thermo-hydraulic control of gas hydrates. The choice of this
particular couple is due to the fact that only this couple provides
for the easy reaction control, high exothermicity, sensitivity to
the medium pH, besides yielding non-corroding by-products.
The buffering system c) consists of an aqueous solution of acetic
acid 40% vol/vol (or more) and the viscosifying agent, whenever
employed, is preferably high-molecular weight hydroxyethyl
cellulose (HEC) which works secondarily as a surface active agent.
One of the advantages of using HEC is that this compound promotes
higher viscosity of the foam on a weight basis of a corresponding
surface active agent. Also, the stability of the foam is improved
when using HEC.
The water used in making the nitrogen salts solution may be any
good quality industrial water, the pH being in the range of 6 to 8.
Preferably, the water is free of ferric ions
The reaction between the nitrogen generating compounds comprises
the steps of reagent dissolution, NH.sub.4.sup.+ NO.sub.2.sup.-
complex formation and further conversion of the complex into
nitrogen gas and water. The rate of the decomposition reaction may
be subjected to variations in the case where the acid hydrogen
species (H.sup.+) is introduced in the medium. Therefore the
equation which describes the reaction rate depends on the
concentration of ammonium, nitrite and acid hydrogen ions. Other
important parameters such as temperature, agitation and viscosity
are associated to the degree of proximity of the chemical species
nitrite and ammonium in the complex formation. On the other hand,
pressure, temperature and solubility regulate the state of the
produced gas (PVT).
The reaction of nitrogen generation indicates an equimolar
stoichiometry between ammonium chloride and sodium nitrite.
Normally the molarity will be between 0.5 and 4.5.
The pH influences the reaction rate, since the reaction mechanism
comprises the effect of the hydrogen ion H.sup.+ in the activated
complex step. Optimum pH values are between 4.75 and 5.50.
The viscosity of the medium affets the reaction rate: the higher
the viscosity, the longer the reaction life time.
While the reagent dissolution is endothermal, the nitrogen
generation is highly exothermic, with release of 70-75 kcal per
mole of consumed reagent, the exothermicity being favorable to the
control of gas hydrates, no matter the circumstances under which
they have been formed: production well, injection well, reservoir
or conduit.
The Nitrogen Generation System as applied in the present invention
may comprise a heat- and nitrogen generating fluid viscosified with
a non-ionic, high molecular weight polymer such as hydroxyethyl
cellulose. This mode is then called SGN/Foam.
The Nitrogen Generating System may equally comprise a heat- and
nitrogen generating fluid in solution. This mode is then called
SGN/Solution.
SGN/Foam Mode
This mode for the control of gas hydrates is mainly applied to the
removal of already formed gas hydrates, for example in a producing
well. However this mode may equally be applied to prevent the
formation of gas hydrate plugs.
FIG. 1 attached is a phase diagram illustrating the displacement of
the gas hydrate to the water+gas phase. Under conditions of high
pressure and low temperature, the water+gas constituents form
hydrate plugs. By applying the SGN/Foam treatment pressure is
reduced and temperature is increased so that the hydrate
constituents return to the water+gas condition.
It is well known that as production goes on in a subsea producing
well, dead oil is separated from gas. When water injection is
initiated, and in the presence of the low subsea temperatures, the
thermo-hydraulic conditions for forming hydrate plugs are
established, and oil production may be hindered.
The mode of the present invention which makes use of the heat- and
nitrogen generating fluid viscosified with a non-ionic polymer
employs the SGN/Foam fluid. Applying this process for example to an
occurrence of gas hydrates for example near a Wet Christmas Tree
(WCT) basically comprises pumping a minimum amount of SGN fluid
previously viscosified with a polymer such as high molecular weight
hydroxyethyl cellulose (HEC) and activated on flow, the fluid being
injected through a flexitube the end of which is positioned
immediately above the top of the hydrate plug, so as to promote
intense foam generation on such plug.
The application of the SGN/Foam technology to a petroleum producing
field is normally preceded by the assessment of the reaction
kinetics at the laboratory scale. Then the obtained data are fed to
a numerical simulator which will design the field operation
procedure from data of composition, volume and flowrate of the
SGN/Foam fluid.
FIG. 2 attached illustrates the basic scheme of the pumping and
circulation operation of SGN/Foam for a subsea production well in
its completion or intervention step. At this step the completion
fluid is within production string (7) in order to control the
hydrostatic pressure of the well. Under the condition of
application of the SGN/Foam fluid there is no flow of oil since the
hydrate plug avoids the flow of oil. Concerning its localization,
according to FIG. 2, the gas hydrate may form in the surroundings
of, that is, above or below the WCT due to the sudden reduction in
temperature as a function of the low temperatures of the seabottom.
On FIG. 2 are shown the production string (7), a flexible conduit
or flexitube (4), a Wet Christmas Tree (9), riser (10) and the gas
hydrate (11) formed.
The nitrogen- and heat generating solution designed to work on the
hydrate blocks or plugs is prepared from an aqueous solution of
NH.sub.4 Cl and NaNO.sub.2 salts in equimolar amounts. The solution
is viscosified with the aid of high molecular weight hydroxyethyl
cellulose (HEC). The thus obtained viscosified fluid SGN (1) is
mixed on flow to the activator (2) so as to yield the activated,
viscosified SGN/Foam fluid (12) which is injected through the
flexitube (4) at the previously established concentration which has
been determined based on kinetic studies.
The activated, viscosified SGN/Foam fluid (12) triggers the foam
generation on flow throughout and up to the lower end of the
flexitube (4), eventually reaching the region of probable formation
of hydrate (11). As a consequence of the released heat and fluid
expansion caused by the SGN/Foam fluid the hydrostatic pressure of
the riser (10) is reduced, the aqueous completion fluid and foamed
fluid being recovered. The increased temperature and reduced
pressure resulting from the SGN/Foam fluid alter the temperature
and pressure conditions which made possible the existence of a
hydrate plug or block so that such plug tends to be converted into
dissociated water and gas. After the injection of the SGN/Foam
fluid oil production should be resumed shortly, between 30 and 60
minutes, so as to avoid the cooling of the components of the
dissolved hydrate.
On meeting the possible gas hydrate plug (11) the SGN/Foam fluid
(12) works according to three different ways:
i) it generates heat through the reaction between the nitrogen
salts, NH.sub.4 Cl and NaNO.sub.2, so as to displace the balance of
the phase diagram to the situation water+gas;
ii) it reduces the hydrostatic pressure by forming a low-density
foam;
iii) the formed foam mechanically removes hydrate portions
This way the thermo-hydraulic conditions which favor the formation
of hydrate plugs or blocks are modified, the plugs or blocks being
dissociated into water and gas so that eventually the SGN fluid
makes that petroleum oil production is resumed.
As a result of the action or treatment with SGN/Foam fluid there is
obtained a spent fluid (13). The spent fluid (13) may be recovered
through two different ways:
i) through the production string (7) itself;
ii) through the space existing between the production string (7)
and the riser (10).
The spent fluid (13) is directed to the oil/gas triphase separator
(14) where the aqueous phase is separated from the oil phase.
One additional advantage of the present process is that the lower
pressure causes surgency to be induced, so that the well can reach
complete flowrate values more rapidly than it would without
injection of the SGN/Foam fluid.
Further, the spent fluid of the SGN/Foam fluid contains salts which
render more difficult that gas hydrate blocks be formed again.
As described hereinbefore, the solution of heat- and nitrogen gas
generation salts is normally constituted by ammonium chloride and
sodium nitrite, the so-called "C+N solution", which is to be
prepared in one single vessel. In order to obtain a foam of
excellent stability, the recommended concentration for the nitrogen
reactants in the SGN/Foam mode may vary between wide limits, but
preferably is between 2.0 and 4.5 molar with 3.0 molar being a
preferred concentration for each of the nitrogen salts.
The aqueous solutions which make up the fluid for the gas hydrate
control are viscosified with high molecular weight hydroxyethyl
cellulose (HEC) in concentration of 0.3 to 1.2 weight/volume. After
adding the viscosifying agent HEC, the pH of the solution is
permanently adjusted to values between 7.4 and 7.7, preferably 7.5,
with the aid of a 50 wt % aqueous solution of sodium hydroxide.
The preparation of the C+N solution comprises, in a typical
case:
______________________________________ Volume of industrial water
0.730 m.sup.3 /m.sup.3 NaNO.sub.2 Concentration 207.0 kg/m.sup.3
NH.sub.4 Cl Concentration 160.5 kg/m.sup.3 Concentration of C + N
solution 3.0 mol/l pH of the C + N solution (adjusted) 7.5 @
25.degree. C. density of the C + N solution 1.15 g/ml @ 25.degree.
C. viscosity of the C + N solution 1.3 cP @ 25.degree. C.
______________________________________
The thus prepared solution is viscosified with a polymeric
viscosifying agent such as hydroxyethyl cellulose HEC. The
viscosifying agent is added to the C+N solution under moderate
agitation at a concentration of 80 lb/gal (9.6 grams/liter),
followed by pH adjustment to values between 8.0 and 8.3 with the
aid of NaOH solution at 50 wt %. For this polymer, the hydration
period is estimated between 2 and 3 hours. The features of the
final solution are as follows:
______________________________________ HEC concentration 9.6
kg/m.sup.3 Volume of NaOH solution at 50 wt % 2.0 liters/m.sup.3
Apparent viscosity 194 cP @ 510 s.sup.-1 Behavior Index 0.28
Consistency Index 154 dyn. s.sup.n /cm.sup.2 final pH of
viscosified C + N solution 8.2 @ 25.degree. C. density of
viscosified C + N solution 1.15 g/ml @ 25.degree. C.
______________________________________
The kinetics of the heat- and nitrogen generation reaction in the
presence of foam has been determined in the labscale based on the
follow-up of foam generation in a gauged cylinder, under ambient
conditions and moderate magnetic agitation, pH, temperature and
volume of foam being simultaneously measured. The C+N solution is
viscosified with HEC at a concentration of 80 lb/gal (9.6
grams/liter or 9.6 kg/m.sup.3); the addition of activator has been
effected immediately after the transfer of the fluid to the gauged
cylinder.
TABLE 1 ______________________________________ Time pH Temperature
Volume.sup.a Yield.sup.b Quality.sup.c (min) (-) (.degree. C.) (ml)
(% vol) (ml/100 ml) ______________________________________ 0 5.1 21
50 0 0 2 5.0 27 400 12 0.875 4 4.9 32 780 23 0.936 6 5.0 34 1050 31
0.952 8 5.0 35 1150 34 0.956 10 5.0 35 1250 37 0.960 14 5.1 36 1380
41 0.964 18 5.2 36 1420 43 0.965
______________________________________
TABLE 2 ______________________________________ Time pH Temperature
Volume.sup.a Yield.sup.b Quality.sup.c (min) (-) (.degree. C.) (ml)
(% vol) (ml/100 ml) ______________________________________ 0 4.9 23
50 0 0 2 4.5 39 1100 32 0.954 4 4.8 47 1950 58 0.974 6 5.1 52 2050
61 0.975 8 5.1 54 2100 62 0.976 10 5.2 56 2150 64 0.977 14 5.2 56
2200 65 0.977 18 5.2 55 2250 67 0.978
______________________________________
a) volume of foam under the test conditions corresponding to an
initial SGN/Foam volume of 50 ml
b) Yield of the reaction of nitrogen gas assuming total
incorporation of the generated gas into the volume of foam
c) Quality of foam as calculated from the equation ##EQU1## wherein
.left brkt-top. is the quality of the foam.
The viability of the present process for the thermo-hydraulic
control of gas hydrates via SGN/Foam is checked by means of a
physical simulation of the generation and circulation of foam. For
such, a labscale physical simulator is assembled according to FIG.
3.
Basically, the physical simulator comprises two long, glass tubes
which are concentrically positioned. The operation of the physical
simulator is as follows:
The viscosified SGN fluid (1) as described hereinbefore is pumped
at constant rate with a varistaltic pump and at the same time
activator (2) is pumped through the same line, at a certain ratio
of activator (2) to viscosified SGN fluid (1). The activated,
viscosified fluid thus obtained is pumped by means of varistaltic
pumps (3) throughout the flexitube (40) from which the fluid may,
in a real field situation, contact the gas hydrate. In the physical
simulator, the viscosified SGN fluid (1) reacts so as to generate a
foam (5) and yield the necessary heat to displace, in the phase
diagram, the hydrate to the water+gas phase. After the reaction is
complete, the returned foam (6) is directed to the production
string (7), and is collected in a proper vessel, a reservoir
(8).
Additional devices such as flexible hoses and glass vessels are
also part of the physical simulator. The pumping of the viscosified
fluid and the activator is duly synchronized and the evolution of
foam as well as the recovery of the original fluid contained in the
simulator (for example, a completion fluid) are monitored during
the test.
The features of the simulation test are as follows:
______________________________________ Overall height of the
simulator 120 cm Volumetric capacity 2100 cm Original fluid in the
simulator Water Volume of SGN/Foam 200 ml Volume of activator (AcOH
50%) 10 ml Pumping rate 100 ml/min Return flowrate 270 ml/min
Volume of recovered fluid 1900 ml Recovery of original fluid 76%
vol/vol Volume of generated foam 6300 ml Yield of foam generation
47% vol/vol ______________________________________
The working of the labscale physical simulator demonstrates that a
viscosified SGN fluid leads to heat and foam which are suitable to
the control of gas hydrates in petroleum producing oils.
Therefore, the present process possesses the required
characteristics which render it suitable to the control of gas
hydrates formed in a production string pertaining to a subsea
well.
The SGN/Foam fluid for the control of gas hydrates according to the
thermo-hydraulic process of the present invention may be applied by
means of a completion rig or a stimulating vessel. The control of
the process is excellent, at the step of the C+N solution
preparation as well as at the step of the simultaneous pumping of
the SGN fluid and the activator with a chromatographic pump, it
should be noted that the content of acetic acid used is different
from that of other applications of SGN in view of the
alkalinization of the nitrogen salts solution at the
viscosification step of the C+N solution.
Also, the physical simulation tests for the process of in situ foam
generation from previously viscosified SGN corroborate the
technical viability of the inventive process, since they indicate
more than 70% recovery of the original fluid. The high quality and
stability fo the generated foam assures a significant pressure
reduction of the hydraulic pressure in the production string. The
step-by-step heat generation helps not only in dissociating the
hydrate plug, but also in avoiding that it be re-formed later
on.
SGN/Solution Mode
In situations such as in the technique of Water Alternating Gas
(WAG) in subsea injection wells, it is common that the
thermo-hydraulic combination for the formation of gas hydrates is
satisfied. This is because the water injection combined to the low
subsea temperatures and to the hydrocarbon gases favor the
conditions to the build up of gas hydrate plugs.
In view of the frequent occurrence of gas hydrate plugs under such
conditions, the control of such plugs may be preventively carried
out using the treating fluid SGN/Solution, where the combination of
heat, nitrogen gas and high salinity of the fluid prevent that gas
hydrate plugs be formed.
For sure, the SGN/Solution fluid may also be used to dissolve
already formed gas hydrate plugs.
A typical application of the SGN/Solution mode is illustrated in
FIG. 4 which represents an injection well submitted to a WAG
treatment which may generate the conditions for gas hydrate
formation.
Broadly, the SGN/Solution treating fluid follows the step of water
injection, is left to react and after a sufficient period of time,
gas is injected according to the WAG system.
In this mode, the nitrogen salts-containing aqueous solution which
will generate nitrogen gas and heat will be used without any other
additive besides the activator. The aqueous solutions of molarity
between 0.5 and 4.5 are prepared in one single mixing vessel,
pumped with the aid of a triplex pump and activated on flow by
means of acetic acid 0.4 to 1.2% vol/vol, for example 1.0% vol/vol
added by means of a controlled-rate pump. From the flexitube (4)
installed in the Wet Christmas Tree (WCT) (9) the activated
SGN/Solution fluid (15) contacts the reservoir through the
perforations (16), the fluid (15) entering then the reservoir and
generating the required heat to prevent the formation of gas
hydrate plugs. So, the SGN/Solution treating fluid creates new
temperature conditions which are preventive to the formation of gas
hydrate plugs, since under these temperature conditions the hydrate
constituents water and gas keep in the dissolved state.
The SGN/Solution treating fluid is equally applicable to the
formation of gas hydrate plugs in conduits or pipelines which
transport hydrocarbons. These plugs may be formed whenever in a
conduit which transports liquid hydrocarbons the gas phase
separates from the liquid. Under conditions of high pressure and
low temperatures, for example in subsea pipelines placed in cold
waters, there is the possibility of hydrate formation.
Conduits transporting gases show the same possibility, any humidity
present in the conduits being able to condense with the gases which
are transported so as to form the gas hydrate plugs.
Numerical Simulation of the SGN/Solution Mode
In an injection well under a water injection pressure of 280
kg/cm.sup.2 (4000 psi) at temperatures of water injection around
10.degree. C. in the penetration radius of the reservoir it is
considered that the conditions for forming a gas hydrate in the
well are met. The WAG technique is widely used in injection wells.
When alternating water injection at low temperatures under
relatively high pressures and gas, it is considered that the
conditions for forming gas hydrates are practically met. It is a
fact that under such thermo-hydraulic conditions the permoporous
properties of the reservoir become impaired. As can be seen in the
phase diagram of FIG. 5 the condition for the dissolution of the
gas hydrate which may be formed must necessarily be obtained from
an increase in temperature, since under such conditions pressure is
a parameter difficult to be altered. Thus, the SGN/Solution
treatment will be mainly directed to provide for the increase in
temperature aimed at preventing the formation of, or dissolving any
hydrates which might be formed in the injection well.
In order to obtain numerical data on an application of the
SGN/Solution in an injection well a STAR software has been
employed. In the area of secondary oil recovery, this software is a
well-known thermodynamic simulator having a chemical-kinetic
component. According to this software, once are defined the
reservoir and the fluid to be injected into the reservoir, data are
provided on the behavior of the injected fluid according to pre-set
conditions.
The STAR simulator employs the concept of volume element or rock
element, each element representing a vertical slice of the
reservoir at a certain distance from the well. The elements are
measured in feet. TABLE 3 below lists, for an injection well, the
results for the simulation of the injection of a volume of 2,000
cubic meters of SGN/Solution fluid of concentration 0.9 mole/liter
at a rate of 4.5 cubic meters/minute, for 7.5 hours. It is assumed
that at a radial distance of 9 ft the influence of the possible
formation of a gas hydrate does not impair the well infectivity.
Therefore, with the use of the SGN/Solution fluid a condition
should be attained where for the element situated radially 9 ft
from the well, the temperature is higher than 20.degree. C. at a
pressure of 280.degree. C., these the thermo-hydraulic conditions
for the gas hydrate formation.
In TABLE 3 below, the expression "event" means:
i) the temperature situation after seawater injection is stopped.
The temperature of the seawater is 10.degree. C. which upon contact
with the well temperature of 80-90.degree. C. reaches 20.degree.
C.;
ii) the temperature situation after stopping the pumping of the
SGN/Solution at 0.9 mole/liter (7.5 hours);
iii) the temperature situation after the soak time for working of
the SGN/Solution fluid (2.5 hours soak time, the total treating
time being 10 hours);
iv) continuous injection of natural gas: after the soak time
natural gas injection is carried out during 1.5 day, the overall
treating time being 48 hours or 2 days. The column indicating 36
hours and 48 hours from the beginning of the fluid treatment shows
the condition of gas hydrate dissolution for the radial distance of
9 ft, since the temperature is than 20.degree. C., a condition
which does not allow the existence of gas hydrate plugs.
TABLE 3 ______________________________________ Temperature
(.degree. C.) 0 h 7.5 h 10 h 12 h 24 h 36 h 48 h event original
situation = end of Radial end of sea pumping distance water of SGN
@ ("soak continued (ft) injection 0.9 Mol/l time") injection of
natural gas ______________________________________ 2 20 84.4 79.4
79.4 79.0 76.1 74.4 3 20 77.2 74.4 74.4 74.4 72.2 71.1 4 20 63.3
62.7 63.3 64.0 65.0 65.0 5 20 46.1 48.3 48.9 50.0 54.4 56.1 6 20
32.8 35.0 36.1 37.2 42.7 45.5 7 20 24.4 26.6 29.2 27.7 33.3 36.1 8
20 21.1 22.2 22.2 22.8 26.1 28.9 9 20 20.0 20.5 20.5 20.5 22.8 22.2
10 20 19.4 20.0 20.0 20.0 21.1 20.0
______________________________________
Data from TABLE 3 show that between 24 and 36 hours after the start
of the SGN treatment it is possible to attain temperatures beyond
the gas hydrate-forming temperatures at a radial distance of 9 ft
in the injection well. Thus, the heat wave generated by the
SGN/Solution treating fluid is transmitted in such a way throughout
the injection well so that after a sufficient soak time the thermal
conditions which would lead to the formation of gas hydrate plugs
no longer exist so that the formation of such plugs is
prevented.
In case the gas hydrate is already formed, the SGN/Solution fluid
may easily remediate the thermal condition. Care should be taken
regarding the control of the injection pressure of the SGN/Solution
fluid since the presence of hydrate alters the reservoir
permeability.
Therefore, the various modes of the treating fluid, that is,
SGN/Foam and SGN/Solution represent a versatile tool for the
preventive as well as the corrective treatment of the various
conditions of formation of gas hydrate plugs, either in producing
wells, injection wells or in pipelines which transport liquid or
gaseous hydrocarbons.
* * * * *